1. Technical Field
The present disclosure described herein relates generally to communication networks and more particularly to synchronization of communication systems.
2. Description of Related Art
Communication systems are known to support wireless and wireline communications between wireless and/or wireline communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks to radio frequency identification (RFID) systems. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireline communication systems may operate in accordance with one or more standards including, but not limited to, Digital Subscriber Lines (DSL) such as Asymmetric DSL (e.g., ADSL2+ represented as International Telecommunication Union (ITU G.992.5)), Very High Speed Digital Subscriber Line 2 (VDSL2; ITU G.993.2), G.fast (Gigabit fast), handshake procedures for digital subscriber line transceivers (G.994.1) and/or variations thereof.
Communication systems as described herein include the transmission of data in the form of packets. In a typical configuration, each packet of data may contain bytes of information at the beginning of the data frame that facilitate synchronization with receiving devices called a preamble or training sequence. In parallel communication channel systems (e.g., DSLs), the preamble can be used to identify distinctive data packets communicated on different parallel channels and therefore the data packet does not require a unique identifier. DSLs represent multi-line, multi-carrier wide-band communication systems. These systems require accurate symbol frame alignment to perform properly and maintain stability. Current DSL implementations, such as ITU G.993.2 VDSL2, utilize common pseudo-random bit sequences (PRBS) for all lines for remote unit symbol frame alignment. In VSDL2, a Frequency Division Duplex (FDD) system, near-end crosstalk (NEXT) generally does not cause synchronization problems while far-end crosstalk (FEXT) impact is relatively low as signal bandwidth is limited to, for example, 17 MHz. However, such an approach can face problems in a Time Division Duplex (TDD) system such as G.fast where signal bandwidth is up to 100 MHz, while loop length is expected to be less than 250 meters. Under such conditions, both NEXT and FEXT can have a strong presence and cause false symbol frame alignment to remote units if an identical synchronization signal is used for all lines.